The Truth About EPA City / Highway MPG Estimates

Where do those mileage figures come from?

Since the late 1970s, the U.S. Environmental Protection Agency has certified the fuel-economy projections of some 450 million new vehicles sold in this country.

The projections are there by law and appear boldly on the window stickers of new vehicles, for example, CITY MPG 16, HIGHWAY MPG 25. They appear authoritatively, almost like a pledge from the federal government, and motorists have put a lot of faith in these numbers.

The EPA figures also determine whether an automaker meets the required fuel-economy averages for a company’s entire vehicle line, so the numbers are a very big deal. As such, you might expect the federal government’s facility to be about the size of, oh, the Department of Agriculture and loaded to the brim with persistent bureaucrats.

While the public mistakenly presumes that this federal agency is hard at work conducting complicated tests on every new model of truck, van, car, and SUV, in reality, just 18 of the EPA’s 17,000 employees work in the automobile-testing department in Ann Arbor, Michigan, examining 200 to 250 vehicles a year, or roughly 15 percent of new models. As to that other 85 percent, the EPA takes automakers at their word—without any testing—accepting submitted results as accurate.

Two-thirds of the new vehicles the EPA does test are selected randomly, and the remaining third are done for specific reasons. We’re not sure why a Porsche 911 GT3 was at the lab when we were there—other than to get an up-close look at its sexy, single-lug wheels—but candidates for scrutiny usually involve new technologies, new manufacturers, class fuel-economy champs, or cars that barely avoid a gas-guzzler tax.

We visited the EPA’s nondescript lab for a look at just how these numbers are derived. Located across the street from the University of Michigan’s North Campus and next to a mundane strip mall, the only clue to its importance is the imposing gate that guards the industrial building complex.

What happens inside is enormously complicated. There are endless reams of documents that explain, in detail, every procedure and circumstance, and each comes with its own set of rules. Even the procedure for rounding off the results of the fuel-economy tests to produce what is published on a new-car label is crazily complex. “We have a guy that’s literally made a career out of specializing in rounding,” says the EPA’s Linc Wehrly, manager of “light-duty compliance,” the man in charge of vehicle testing.

So how does the EPA lab work?

Vehicles are tested on dynamometers, or dynos, which are like giant treadmills for cars. The vehicle is held stationary while its wheels spin the dyno’s large rollers. There are just three dynos, and only one of them is a four-wheel-drive unit with sets of rollers for both the front and rear wheels; the two other dynos are spun by only a car’s driven wheels. The four-wheel dyno was added fairly recently; before that, all testing had to be done on two-wheel-drive dynos, which necessitated the additional complexity of disconnecting driveshafts on AWD models so they could be converted to two-wheel drive. (How weird can it get? The EPA created a two-wheel-drive version of the $1.7 million, 1001-hp Bugatti Veyron—the world’s fastest and most outrageous production car—for this purpose, prompting visions of burnouts of nuclear dimension.) When tested this way, additional drag is applied to the dyno to replicate normal operation of the AWD system.

Every last detail of an EPA fuel-economy test has specific rules—there’s even a set volume of air that a fan must blow under the car’s raised hood. “Driving” the test by matching the red line is tricky and takes a sensitive foot.

After a vehicle is strapped down on a dyno, the staff punches in coefficients that allow the dyno rolls to simulate real-world factors, such as wind and road friction.

One of the EPA’s six drivers is behind the wheel of the test car. With an average of 20 years’ experience, they have extremely fine-tuned throttle and brake-control skills. They “drive” by following a precise red line of speed versus time that’s displayed on a monitor hanging just in front of the windshield. Using the gas pedal and the brakes, the driver attempts to match the red line with the car’s wheel speed, which is shown in white. We got into a test car and tried it, and indeed, it’s very difficult to maintain the speed of the tests, particularly when it dithers in the single digits and a brush of the throttle can send the white line careening off-course. If the speed deviates from the test cycle by more than 2 mph, the results are thrown out. For manual-transmission cars, there are standard EPA shift points, which are broadcast on the driver’s screen as well.